The present invention relates to manufacturing thermal shielding elements intended most particularly for structures that are liable to be subjected to very high surface temperatures, typically greater than 1000.degree. C., and possibly as high as 1800.degree. C.
A non-exclusive field in which the invention can be applied is the manufacture of thermal shielding elements for space planes. On re-entering the atmosphere, space planes are subjected to major surface heating, and the surface temperature of certain potions thereof, e.g. the nose and the leading edges of the wings, may reach 1300.degree. C. to 1400.degree. C.
Known thermal shielding is generally of types: ablative shielding where thermal energy is absorbed by the substance constituting the shielding being progressively destroyed; and radiant shielding that dumps thermal energy by radiating it.
The drawbacks of ablative shielding include being unsuitable for reuse and giving rise to a change of shape while in use. That is why radiant shielding capable of withstanding high temperatures is used in applications such as space planes.
Thus, it is well known that the surface of a space plane to be shielded can be covered in tiles or blocks of insulating ceramics material. The tiles are stuck on with an interposed layer of material that forms an adaptor for accommodating relative deformation between the tiles and the cold structure to be shielded. That solution suffers from several drawbacks.
Tiles of ceramics material are sensitive to impact, and there is a high risk of chipping. Furthermore, they cannot always withstand the deformation of the cold structure that carries them, in spite of the presence of the material for accommodating such deformation. The technique of using an adhesive to secure the tiles is difficult to implement and not very reliable, and the operations of removing and replacing damaged tiles are lengthy and difficult. In addition, it is necessary to impart a shape to the cold supposing structure that reproduces the desired aerodynamic shape, which is itself defined by the tiles that are stuck to the cold structure and that perform the functions both of insulation and of fairing or streamlining.
To avoid those drawbacks, proposals are made in document FR-A-2 657 675 to use a thermal shielding element comprising a panel of thermal structural composite material in the form of a shell filled with thermally insulating material and suitable for being fixed to a cold supporting structure by means of mechanical fastening members. The panels are juxtaposed with interposed gaskets or jointing to form a substantially continuous outside surface, with access to the mechanical fixing members being possible from the outside by deforming the jointing.
With such a design, the functions of providing streamlining, insulation, and mechanical strength for the supporting structure are decoupled. The streamlining is defined by the panels without it being necessarily reproduced by the supporting structure, whereas insulation is provided, at least in part, by the insulating material inserted between the panels and the supporting structure.
In addition, the mechanical fixing members enable connections to be made by means of screws, thereby making it possible to do without adhesive and greatly facilitating the removal and replacement of a panel.
Finally, panels made of a thermostructural composite material, e.g. a ceramic matrix composite material, withstand impact better than do solid ceramics. In particular, the risk of chipping is practically non-existent.
Nevertheless, the manufacture of panels made of thermostructural composite material is lengthy, difficult, and expensive.
Thermostructural composite materials, i.e. composite materials whose mechanical properties make them suitable for constituting structural elements and which retain those properties up to high temperatures, are typically carbon-carbon (C--C) composite materials constituted by a reinforcing fabric or preform of carbon fibers densified by a carbon matrix, or else ceramic matrix composite (CMC) materials constituted by a reinforcing fabric or preform of refractory fibers (fibers of carbon or of ceramic) densified by a ceramic matrix.
To manufacture a panel of thermostructural composite material, a fiber preform is made initially, e.g. by draping plies of cloth, where the number of superposed plies is selected as a function of the thickness desired for the panel. The plies are draped over a tooling element whose shape reproduces the shape of the panel to be manufactured.
The panel is densified by liquid means (impregnation) or by gaseous means (chemical vapor infiltration). When using a liquid, the fiber preform is impregnated by a liquid precursor of the matrix material, with subsequent transformation of the precursor generally being obtained by heat treatment. When using a gas, the fiber preform is placed in an enclosure into which a gas is admitted that, under predetermined conditions of temperature and pressure, forms a deposit on the fibers inside the preform, either by decomposing or else by means of a reaction between the components of the gas. The above densification techniques making use of a liquid or of a gas for forming a matrix of carbon or of ceramic are well known.
During the densification process, it is often necessary to maintain the fiber preform in the desired shape, and that requires the use of tooling. When the shape of the product to be made is complex, as in the case of the shell-shaped panels mentioned above having portions for linking with fixing members, it is necessary to make use of special tooling. In addition, the tooling must be made of a material that is capable of withstanding the temperatures that are reached during the densification process, while also being inert relative to the materials constituting the preform, the matrix, and precursors therefor. Typically, for densification by means of chemical vapor infiltration, graphite tooling is used which is heavy, bulky, and very expensive.
In addition, after the panels have been made, it is necessary to install the thermal insulant with which they are filled. Unfortunately, the thermal insulant disclosed in the above-mentioned FR-A-2 657 675 is an insulant of the multiple screen type, made up of a stack of metal plated ceramic sheets. Such an insulant is fragile, very difficult to implement in receptacles of complex shapes, and it is sensitive to moisture.